Knife sharpener



FIG. 1 is a front elevational view of a knife sharpener showing my new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a left side view thereof;

FIG. 4 is a right side view thereof;

FIG. 5 is a top plan view thereof;

FIG. 6 is a bottom plan view thereof; and,

FIG. 7 is a perspective view thereof.

The broken lines depict portions of the knife sharpener in which the design is embodied that are not considered part of the claimed design. 

CLAIM The ornamental design for a knife sharpener, as shown and described. 